Currently, as substrates for solar cells, silicon crystal is chiefly used. Such silicon crystal is divided into single crystals and polycrystals, and solar cells using single crystal silicon as a substrate have higher conversion efficiency for converting energy of incident light into electrical energy than ones using polycrystalline silicon as a substrate.
Single crystal silicon is produced as a dislocation-free high quality crystal for example by Czochralski process. However, when single crystal silicon is produced by Czochralski process, the production cost is high and such single crystal silicon is not practical being used as a substrate for solar cells. It is therefore common to produce solar cells using polycrystalline silicon which can be cast from inexpensive materials.
One of the methods of casting such polycrystalline silicon is electromagnetic casting (for example, see JP 2007-051026 A (PTL 1)). Electromagnetic casting is a method in which a silicon raw material in a crucible is heated and melted by high frequency induction, meanwhile molten silicon is suspended by the action of strong electromagnetic force, thereby continuously growing an ingot. On this occasion, the molten silicon is not contact with the crucible, so that a high quality ingot can be cast. A polycrystalline silicon wafer obtained from a thus cast polycrystalline silicon ingot is characterized by enabling the production of solar cells that have high uniformity in quality in the casting direction, less variation in the conversion efficiency, and stable characteristics.
In order to increase the conversion efficiency of solar cells, it is important to appropriately control impurities such as oxygen, carbon, and nitrogen that are contained in a polycrystalline silicon wafer. Among others, carbon promotes the precipitation of oxygen and the precipitated oxygen serves as dislocation multiplication sources. The formed dislocations serve as recombination centers of carriers, which results in reduced conversion efficiency of solar cells; thus, it has been considered that the lower the carbon content is in the wafer, the better. For example, WO 2006/104107 (PTL 2) describes a technique of controlling the carbon concentration to 1×1017 atoms/cm3 or less (the interstitial oxygen concentration is 2×1017 atoms/cm3 or more), thereby realizing high conversion efficiency of solar cells.